1. Field of the Invention
The invention relates to electrochemical devices, and in particular electrically controllable systems with variable optical and/or energetic properties of the electrochromic glazing or mirror type.
2. Discussion of the Background
In a known manner, electrochromic systems comprise a layer of an electrochromic material capable of reversibly and simultaneously inserting ions and electrons and whose oxidation states corresponding to the inserted and de-inserted inserted states are of distinct colourization, one of the states presenting greater light transmission than the other, the insertion or de-insertion reaction being controlled by a suitable electrical supply. The electrochromic material, usually based on tungsten oxide, must thus be placed in contact with a source of electrons such as a transparent electrically conductive layer and a source of ions (cations or anions) such as an ionic conductive electrolyte.
Moreover, it is known that in order to ensure at least a hundred switching operations, the layer of electrochromic material must be combined with a counterelectrode also capable of reversibly inserting ions, symmetrically relative to the layer of electrochromic material, such that, macroscopically, the electrolyte appear as a simple medium of ions.
The counterelectrode must consist of a colour-neutral layer, or a layer which is at least transparent or only slightly coloured when the electrochromic layer is in the coloured state. Since tungsten oxide is a cathodic electrochromic material, i.e. its coloured state corresponds to the most reduced state, an anodic electrochromic material based on nickel oxide or iridium oxide is generally used for the counterelectrode. It has also been proposed to use a material which is optically neutral in the oxidation states concerned, such as, for example, cerium oxide or organic materials such as electron-conducting polymers (polyaniline, etc.) or Prussian blue.
A description of such systems will be found, for example, in European patents EP-A-0 338 376, EP-A-0 408 427, EP-A-0 575 207 and EP-A-0 628 849.
These systems can currently be classified in two categories, depending on the type of electrolyte they use:                either the electrolyte is in the form of a polymer or a gel, for example a proton-conducting polymer such as those disclosed in European patents EP-A-0 253 713 and EP-A-0 670 346, or a lithium-ion-conducting polymer such as those disclosed in patents EP-A-0 382 623, EP-A-0 518 754 or EP-A-0 532 408,        or the electrolyte is a mineral layer, which conducts ions but is electronically insulating, such systems being referred to as “all-solid” electrochromic systems. For the description of an “all-solid” electrochromic system, reference may be made to European patent applications EP-A-0 867 752 and EP-A-0 831 360.        
The present invention is directed most particularly towards obtaining layers of anodic electrochromic material based on nickel oxide which are capable of forming a part of such electrochromic systems.
As mentioned above, nickel oxide is known to have such a property, and is described as such in particular in patent EP-0 373 020 B1.
However, this material has a drawback: certain difficulties arise in obtaining it in the form of a thin layer by a standard vacuum deposition process, magnetic-field-assisted reactive cathodic sputtering: since nickel is ferromagnetic, using a standard nickel target and a standard magnet, the magnetic field developed at the surface of the target is weak, resulting in a low deposition rate and mediocre exploitation of the target.
This type of material was also studied in patent application WO 98/14824: in an application to electrochromic mirrors, studies were carried out on nickel oxides alloyed with another metal such as vanadium, chromium, manganese, iron or cobalt, this change of composition being said to improve the functionality of the mirror, and in particular to give it a more uniform colour.
However, introducing other metals into the nickel oxide in this way appears to entail risks as regards the optical and electrochemical properties of the nickel oxide. Thus, it may be feared, for example, that the introduction of vanadium and chromium, the oxides of which absorb in the visible region, tends to make the nickel oxide more absorbent and, consequently, tends to reduce the light transmission value of the active system as a whole when it is in the decolourized state. Similarly, the introduction of manganese, iron and cobalt may tend to lower the durability of the layer and thus of the active system as a whole.